I can't think of much as a post-war design that's attractive enough to want to collect. There have been some very successful stationary engines since, but not for me. Will I ever get into Honda generators??
Strangely, in the moving engines world (as opposed to stationary) there have been all manner of wonderful engines since the war.
Masses of splendid innovative Italian designs with art-quality castings; many great Japanese & continental multi-ohc jobbies; Wankels; some stunning aircraft engines. I've got a variety of these -- mainly wrapped with chassis by Laverda, Ducati, Gilera, Capriolo & a few others. I've resisted the aircraft so far .....
Some of us like technical innovation, some like landmark social impact of our engines, some (including me ...) just like stationary engines where you see all the big bits working slowly
Few stationaries designed since 1950 meet ground-breaking & universally interesting technical innovation criteria, & very few are aesthetically pleasing. Ergo, not much attraction to me.
Mind you, would you have expected folk to collect Bond Minicars or NSU Quickly's? ......... so there's a home for most rusty relics. Colin Osborne